FIELD OF THE INVENTION
This invention relates to helicopter rotors having a plurality of blades which move substantially in a common plane of track and more particularly to means for adjusting individual rotor blades to correct for lead/lag and flap track errors.
A helicopter rotor typically includes two or more blades which extend from a hub. For a particular type of rotor head such as a bearingless main rotor (BMR), each blade typically has associated with it a flexible spar which extends from the hub. The spar is used to connect the blade to the hub. A torque tube surrounds the spar and has a control rod attached to it which allows a pilot to change the blade pitch. A pair of snubber dampers are disposed between the spar and the torque tube to support the torque tube in proper relationship with the spar. The snubber dampers transfer push rod loads from the torque tube through the snubbers into the spar.
A snubber damper consists of a series of spherical (snubber) and flat (damper) shims separated by elastomeric layers. The two snubbers located above and below the spar combine to form a spherical bearing which is used to support and position the torque tube in a proper relationship to the spar for limiting pitching and flapping motions. The damper portions combine to provide sufficient damping of blade lead/lag motion to maintain rotor stability.
As the rotor turns, the blades define a circular track. Due to the bolted attachment of the blades to the hub, or slight differences in manufacturing tolerances, such as in the spring rate of the snubbers, it is possible for tracking errors to occur. Track errors may be exhibited by a change in spacing between adjacent blades, i.e., a lead/lag error, or out of the plane of the track, i.e. a flapping error. Even slight variations at the hub can translate to large variations at the blade tips. The result of these variations is that the rotor acts as an unbalanced assembly with consequent vibration. It is beyond present manufacturing capabilities to eliminate these variations.